1. Field of the Invention
The present invention relates to a method for recording sector control information onto a high density magneto-optical disk.
2. Description of the Prior Art
On the disk medium used as an external data storage device of computers, the track area is divided into sectors S of an appropriate length each for ease of data handling and data access, as shown in FIG. 1. Sector control information such as physical addresses on the disk D is recorded in each sector S so that data may be manipulated in units of sectors. The example of FIG. 1 depicts a CAV (constant angular velocity) type disk.
As outlined, each track of the disk D is usually divided into dozens of sectors for use. In the case of the magnetic disk, sector control information for each sector is recorded upon initialization and prior to the recording of data. For the optical disk, sector control information is written thereon in advance as embossed signals (pits made by pressing). The mode of storing sector control information as embossed signals also applies to the magneto-optical disk which permits subsequent recording of data.
FIG. 2 illustrates an ISO-standard sector format for the WO (write once) optical disk and MO (rewritable magneto-optical) disk. As shown, one sector is composed of a header part HD and a recording data part DA. The header part HD is recorded in advance as an embossed signal (pre-pits) onto the magneto-optical disk medium. The header part HD comprises a sector synchronizing part and an address part. The sector synchronizing part is used to provide a distinct demarcation between two sectors. The address part contains sector control information such as the physical address of the current sector on the disk. The physical address is illustratively composed of a track address and a sector address. In some cases, the physical address is a serial number representing the sector.
As described, the header part HD is conventionally recorded in advance as pre-pits. The header part HD is usually recorded at the same track recording density and using the same modulation method as the recording data part.
For example, the rewritable magneto-optical disk has a magneto-optical recording film. A laser beam is irradiated at the film for heating thereof. This causes the magnetized direction (i.e., recording pits) of the irradiated spot to reflect the externally applied magnetic field representing data. For reproduction, the laser beam is irradiated at a track of the recording pits. The reflected light has its plane of polarization rotated depending on the magnetized direction, a phenomenon known as the Kerr effect. Where the magneto-optical disk has a multiple layer structure such as one containing a reflection film in addition to the magneto-optical film, the Faraday effect is also applicable.
The track recording density at which information is recorded onto the magneto-optical disk is determined by the C/N (carrier to noise) ratio of a given reproduction signal. In the typical prior art setup for recording and reproducing data to and from the magneto-optical disk, a laser beam is irradiated at the disk surface to form a beam spot 1 thereon. The entire beam spot 1 provides a region from which to detect a reproduction signal, as shown in FIG. 3. Thus the reproducible track recording density is determined by the diameter of the laser beam spot.
For example, as depicted in FIG. 3(A), when the diameter d of the laser beam spot 1 is smaller than the pitch .tau. of recording pits 2, there is no possibility of two recording pits being included within the spot 1. In this case, the reproduction signal is read properly from the disk, with the reproduced output waveform occurring as shown in FIG. 3(B). If the recording pits are formed at a higher density, as in the case of FIG. 3(C) wherein the diameter d of the laser beam spot 1 is greater than the pitch .tau. of the recording pits 2, two recording pits are simultaneously covered by the spot 1. This results in a reproduced output waveform occurring as a substantially flat pattern, as depicted in FIG. 3(D), and no signal can be reproduced.
The spot diameter d depends on the wavelength .lambda. of the laser beam and on the numerical aperture (NA) of the objective lens used. Conventionally, the spot diameter d of the laser beam is reduced for higher recording density either by shortening the wavelength .lambda. of the laser beam or by increasing the NA of the objective lens. However, constraints posed by the construction of the laser source and optical system prevent such measures from achieving significantly higher recording densities.
The track density is primarily limited by the cross talk emanating from adjacent tracks. In the conventional setup, the amount of the cross talk also depends on the laser beam spot diameter d. This is another constraint on the effort to enhance recording density.
It was under such circumstances that this applicant proposed some time ago a magneto-optical disk and a method for reproducing data therefrom, the disk being constructed in such a manner that the readable track recording density and track density thereof were maximized without changing the laser beam spot diameter.
One aspect of the proposed method involves the use of a magneto-optical disk having a multiple layer structure comprising a recording layer 3, an intermediate layer 4 and a reproduction layer 5, as shown in FIG. 4(A). The Curie temperature involved is 300.degree. C. for the recording layer 3, 120.degree. C. for the intermediate layer 4, and 400.degree. C. or higher for the reproduction layer 5.
With this magneto-optical disk, the recording layer 3, intermediate layer 4 and reproduction layer 5 are coupled magnetostatically or on a magneto-optical switching basis at room temperature before reproduction, as shown in FIG. 4(A). The recording pits of the recording layer 3 are all transcribed to the reproduction layer 5. The arrows in the figure indicate the directions of magnetization in the respective layers.
Upon reproduction, a laser beam 6 is irradiated at the magneto-optical disk under a reproducing magnetic field Hre, as depicted in FIG. 4(B). As shown in FIG. 4(C), the irradiation of the laser beam 6 produces in the intermediate layer 4 a domain whose temperature exceeds its Curie temperature. Meanwhile, the magneto-optical disk is rotating at high speed. Thus the high temperature domain 8 shifts in the rotating direction away from the position of the scanning spot 7 of the irradiated laser beam 6, as depicted in FIG. 4 (C). The amount of the shift corresponds to the linear speed of the magneto-optical disk.
In this high temperature domain (mask domain) 8, the temperature of the intermediate layer 4 is higher than Curie temperature Tc. Thus the magnetic property of that intermediate layer 4 is lost, as shown in FIG. 4 (B). In turn, the magnetic connection disappears between recording layer 3 and reproduction layer 5 within the domain 8. The magnetized direction of the reproduction layer 5 coincides with the reproducing magnetic field Hre. That is, the recording pits of the reproduction layer 5 are erased from the high temperature domain 8. The scanning spot 7 minus its portion overlapping with the domain 8 leaves out a part 9 that provides an actual reproduction region. In other words, the high temperature domain 8 masks part of the scanning spot 7 of the laser beam, leaving a smaller, unmasked part to form the reproduction region 9.
Under the scanning spot 7 of the laser beam, the smaller, unmasked reproduction region 9 reflects a light beam. The Kerr rotation angle of the reflected light is then detected for pit reproduction. This is equivalent to reducing the diameter d of the laser beam spot 7, which enhances track recording density.
The applicant proposes to call the data reproduction method described above an erasure type data reproduction method. The nomenclature will be used hereunder in this specification.
This applicant has also proposed a second method for data reproduction from the magneto-optical disk. This method is disclosed in Japanese Patent Application No. 1-229395.
According to the applicant's second method, the magneto-optical film of the disk is a laminated film principally comprising a recording layer and a reproduction layer. The recording layer and reproduction layer are coupled magnetostatically or on a magneto-optical switching basis. The Curie temperature of the reproduction layer is lower than that of the recording layer. At room temperature, the contents of the recording layer are magnetically transcribed to the reproduction layer.
This method works as follows. An initializing magnetic field is applied to the magneto-optical disk before reproduction. This orients the direction of magnetization of the reproduction layer in alignment with the initializing magnetic field, erasing the recording pits from the reproduction layer. The magnitude of the initializing magnetic field Hin is greater than that of the magnetic field Hcp for reversing the magnetized direction of the reproduction layer (Hin&gt;Hcp), and is sufficiently smaller than the magnetic field Hcr for reversing the magnetized direction of the recording layer (Hin&lt;&lt;Hcr).
For reproduction, a laser beam is irradiated at the magneto-optical disk in the above-mentioned initialized state. As with the erasure type data reproduction method, the scanning spot shifts in the disk rotating direction by the amount reflecting the linear speed of the disk. The shifted portion (corresponding to the domain 8 of FIG. 4c) has a disk temperature higher than a predetermined temperature Ts. This reduces the coercive force of the reproduction layer under that portion. As a result, the recording pits of the recording layer are transcribed only to the reproduction layer of the portion whose temperature is higher than the predetermined temperature Ts; the recording pits are thus embossed into the reproduction layer. Part of the data-embossed domain overlaps with the laser beam spot. The overlapping part reflects a light beam. From the reflected light, the Kerr rotation angle of the plane of polarization is detected for data reproduction.
With the second method above, that part of the laser beam scanning spot which is not covered by the data-embossed domain whose temperature is higher than the predetermined temperature Ts, is called a masked domain; no recording pits appear in the masked domain. The overlapping part between data-embossed domain and beam spot becomes the reproduction region. Because this region is smaller in diameter than the beam spot, track recording density is enhanced in the same manner as with the erasure type data reproduction method.
With the second method, a reproduction region 18 is an overlapping part between the spot 16 and a data-embossed domain 17 smaller than that spot. Thus the reproduction region 18 becomes smaller in area than the spot 16 in the radial direction of the disk as well. This means that this method additionally permits track density to be raised.
In practice, a four-layer magneto-optical film is formed on the disk, as illustrated in FIG. 5. This film structure is adopted for two purposes: to stably maintain the initial state of the reproduction layer, and to transcribe recording pits properly from the recording layer upon reproduction.
As shown in FIG. 5, the magneto-optical disk according to the second method comprises a four-layer laminated film made of a recording layer 11, an intermediate layer 12, a reproduction support layer 13 and a reproduction layer 14. The Curie temperature involved is 250.degree. C. for the recording layer 11, 250.degree. C. for the intermediate layer 12, 120.degree. C. for the reproduction support layer 13, and 300.degree. C. or higher for the reproduction layer 14.
The recording layer 11 is a layer that contains recording pits free of the effects of initializing magnetic fields, reproducing magnetic fields or reproducing temperature. This layer retains sufficiently high coercive force at room temperature and at a reproducing temperature Ts.
The vertical anisotropy of the intermediate layer 12 is smaller than that of the reproduction support layer 13 or of the recording layer 11. For this reason, when a magnetic domain wall is formed between reproduction layer 14 and recording layer 11, that wall resides stably in the intermediate layer 12. This allows the reproduction layer 14 and reproduction support layer 13 to maintain their erased state (initial state) stably.
The reproduction support layer 13 is provided to reinforce the coercive force of the reproduction layer 14 at room temperature. This in turn stabilizes the reproduction layer 14 and reproduction support layer 13 in their direction of magnetization regardless of the presence of magnetic domain walls. Upon reproduction, the reproduction support layer 13 has its coercive force abruptly reduced near the reproducing temperature Ts. This allows the magnetic domain wall, contained within the intermediate layer 12, to expand into the reproduction support layer 13, ultimately reversing the reproduction layer 14 in its magnetized direction and thereby eliminating the magnetic domain wall. These steps cause recording pits to appear in the reproduction layer 14.
The reproduction layer 14 has a low reversible magnetic field Hcp even at room temperature and is subject to easy reversal of its magnetized direction. This means that the initializing magnetic field Hin orients the entire reproduction layer 14 in one direction of magnetization. Thus oriented, the magnetized direction of the reproduction layer 14 is kept stable, supported by the reproduction support layer 13 even if there exists a magnetic domain wall against the recording layer 11. As described, the magnetic domain wall between reproduction layer 14 and recording layer 11 disappears upon reproduction, allowing the recording pits to appear in the reproduction layer 14.
Prior to actual reproduction, the initializing magnetic field Hin initializes the reproduction layer 14 and the reproduction support layer 13, as illustrated in FIG. 6(A). At this point, the magnetic domain wall (indicated by horizontal arrows in FIG. 6) resides stably in the intermediate layer 12. The reproduction layer 14 and reproduction support layer 13 stably maintain their initialized state.
Next, as shown in FIGS. 6(B) and 6(C), a laser beam 15 is irradiated at a track of recording pits under a reproducing magnetic field Hre. The magnetic field Hre needs to be high enough to reverse the reproduction layer 14 and reproduction support layer 13 in their magnetized directions and to remove the magnetic domain wall from the intermediate layer 12 at a reproducing temperature Ts after the temperature rise following laser irradiation. The reproducing magnetic field Hre should not be high enough to reverse the reproduction support layer 13 entirely in its magnetized direction.
The temperature rise following irradiation of the laser beam 15 produces a data-embossed domain 17 in the magneto-optical disk, as described. This domain 17 is shifted in the disk rotating direction away from the beam scanning spot 16 and has a temperature higher than the reproducing temperature Ts. In the data-embossed domain 17, the coercive force of the reproduction support layer 13 (shown shaded in FIG. 6(C)) is reduced. Because the reproducing magnetic field Hre is smaller than the switching connection force between recording layer 11 and reproduction layer 14, the magnetic domain wall disappears from this portion of the reproduction support layer 13. This triggers transcription of the recording pits from the recording layer 11 to the reproduction layer 14; the recording pits thus appear in the reproduction layer 14. Of the scanning spot 16, a portion 18 which overlaps with the data-embossed domain 17 becomes an actual reproduction region. That is, all area of the laser beam scanning spot 16 excluding the portion 18 overlapping with the data-embossed domain 17 is masked. The overlapping portion 18 thus turns into the reproduction region.
The small reproduction region 18 where the laser beam scanning spot 16 overlaps with the data-embossed domain 17 reflects a light beam. From the reflected light, the Kerr rotation angle is detected so as to reproduce the recording pits. This is equivalent to reducing the diameter d of the laser beam spot 16, which enhances both track recording density and track density.
The applicant proposes to call the above-described second method a data-embossed type reproduction method. The nomenclature will also be used hereunder in this specification.
As described and according to the erasure type data reproduction method and data-embossed type reproduction method, the track recording density and track density involved may be enhanced without the need to reduce the diameter of the laser beam scanning spot.
However, the above-described two methods have experienced troubles in connection with the recording of the header part HD containing sector control information. When the header part HD is recorded beforehand on the magneto-optical disk as an embossed signal, neither method can be used to reproduce the header part. Where the header part HD is recorded at the same track recording density as the recording data part DA, the header part HD cannot be reproduced properly.